1. Field of the Invention
The present invention relates to a device and method for forming an explosive charge and explosive from detonating cord and for initiation of receptors such as signal transmission lines and explosive charges.
2. Related Art
In prior art explosive initiation systems it is known to lower into a borehole a cast booster explosive having a cap well into which has been inserted an electric detonator. The electric detonator is fitted with electrically conductive legwires which are long enough to extend from within the borehole to the surface of the blasting site. The long legwires of such systems are expensive and subject to breakage in lowering and positioning the cast boosters in the borehole. In addition, the assembly of the primary explosive of the detonators with the secondary explosive cast boosters in the borehole increases the handling risks relative to boosters that do not contain primary explosive materials.
It is also known in the art to utilize, in lieu of the electrically conductive legwires, downline high-energy detonating cords to initiate the cast booster explosives. Such high-energy detonating cords typically have explosive core loads from about 3.8 to 10.6 grams per linear meter of cord (xe2x80x9cg/mxe2x80x9d), equivalent to 18 to 50 grains per linear foot of cord (xe2x80x9cgr/ftxe2x80x9d) of pentaerythritol tetranitrate (xe2x80x9cPETNxe2x80x9d) or equivalent amounts, in terms of explosive power, of other secondary explosive. Such high-energy detonating cord is used in the mining industry to initiate the cast booster explosives without the intervention of a detonator between the downline detonating cord and the cast booster. In mining operations, however, the high-energy detonating cord tends to disrupt the bulk (main) explosive charge and is expensive as compared to low-energy detonating cord. In seismic blasting operations, the use of high-energy detonating cord is not satisfactory because the high-energy detonating cord releases significant energy along paths remote from the points at which energy is released by the cast booster charges, and therefore renders seismic data less precise.
It is also known to utilize low-energy detonating cord to directly (without an intervening detonator or the like) initiate an explosive charge which contains a sensitive explosive against which the low-energy detonating cord is placed and which is in contact or close proximity with an explosive charge comprising a less sensitive, e.g., secondary, explosive. This arrangement requires utilizing a more sensitive explosive in conjunction with a less sensitive one, thereby increasing the risk of accidental initiation of the explosive charge.
U.S. Pat. 5,714,712, issued to Ewick et al, discloses an explosive initiation system which ameliorates many of the problems discussed above by directly connecting a low-energy detonating cord to the booster explosive. The system of U.S. Pat. No. 5,714,712 is especially useful for initiating a plurality of substantially simultaneous seismic detonations and includes an electric trunkline circuit disposed on the surface of a firing site containing boreholes, within which booster charges are disposed. The booster charges 30a-30d (FIG. 1 of U.S. Pat. No. 5,714,712) are connected without intervening detonators to the downhole ends of equal-sized lengths of low-energy detonating cord 28a-28d, the surface ends of which are connected to electric detonators contained within connector blocks 24a-24d, which are connected in series in the firing circuit.
FIG. 2 of U.S. Pat. No. 5,714,712 illustrates one way of connecting the downhole end of the low-energy detonating cord 28a to a booster charge 30a by embedding a knotted end of the low-energy detonating cord within the cast booster charge 30a. The knot renders the cord in a non-cylindrical, non-planar configuration. The embodiment of FIG. 2 requires factory manufacture to cast the explosive around the knotted low-energy detonating cord and precludes onsite cutting of the detonating cord to selected lengths from a spool. In the embodiment illustrated in FIGS. 2A and 2B, a cord retaining member 41 is used to retain a double length of the low-energy detonating cord within a cord well 39 formed in the top portion 32x of the cast booster charge 30x. The embodiment of FIGS. 2A and 2B may be assembled in the field but can expose only a limited amount of low-energy detonating cord to the booster explosive.
As used herein, the term xe2x80x9cdetonating cordxe2x80x9d has its usual meaning of flexible, coilable cord having a core of high explosive, the core being a secondary explosive, usually PETN. The term xe2x80x9clow-energy detonating cordxe2x80x9d or xe2x80x9cLEDCxe2x80x9d, is conventionally used to mean detonating cord which will not reliably initiate itself when placed in contact with itself by coiling or crossing lengths of the cord, and which will not, when in an ungathered configuration, reliably directly initiate a less sensitive or secondary explosive receptor charge, e.g., those that comprise secondary explosive materials (e.g., Pentolite mixtures of PETN and trinitrotoluene (xe2x80x9cTNTxe2x80x9d)) to the substantial exclusion of primary explosive materials. Such ungathered configurations include, e.g., simple surface-to-surface contact between an uncoiled LEDC and a receptor charge and the insertion of the end of a substantially straight length of LEDC into a bore in the body of a receptor charge. For this reason, LEDC is typically used to initiate a more sensitive, high energy amplifying device such as a detonator which is sensitive to the LEDC (usually by virtue of containing a primary explosive material) and which generates an output signal sufficient to initiate the less sensitive secondary explosive receptor charge.
The present invention provides a method for forming an explosive charge, the method comprising forming a length of detonating cord into a substantially helical coil comprising a plurality of windings with a cut-off barrier between adjacent windings.
According to various aspects of the invention, the method may comprise spacing adjacent windings not more than about 0.5 inch (12.7 mm) from each other, e.g., about 0.13 inch (3.3 mm), the method may comprise wrapping the detonating cord about a spindle which may optionally comprise the cut-off barrier; the method may comprise forming the length of detonating cord in a tapered coil which may optionally define a taper angle of from about 2 to 4 degrees; or the method may comprise forming the length of detonating cord in a cylindrical coil.
According to another aspect of the invention, the detonating cord may have a core of explosive material with a loading of less than 15 grains per foot of the cord. For example, the detonating cord may have a core of explosive material with a loading of 12 grains or less per foot of the cord, or a loading in the range of from 8 to 12 grains per foot of the cord.
According to still another embodiment of the invention, the coil may comprise about six inches of detonating cord.
This invention also provides a method for forming an explosive charge comprising forming a length of detonating cord in a substantially planar spiral comprising a plurality of windings. Optionally, the detonating cord in the spiral may have a core of explosive material with a loading of at least 2.5 grains per foot of the cord.
The invention also provides an explosive charge comprising a length of detonating cord as described above disposed in a substantially helical coil or planar spiral configuration by the foregoing method or by any other means.
According to one aspect of this invention, the initiator may comprise a spindle about which the coil is disposed. The spindle may optionally be configured to support a substantially helical coil that defines a taper angle of from about 2 to 4 degrees. The spindle may optionally comprise the cut-off barrier.
Alternatively, the spindle may be configured to support a substantially planar coil. In such an embodiment, the detonating cord may have a core of explosive material with a loading of at least 2.5 grains per foot of detonating cord, optionally at least 15 grains per foot. The spindle may comprise a pair of plates between which the substantially planar spiral is disposed.
This invention also relates to a method for initiating an explosive receptor charge. The method comprises inserting into the explosive charge an initiator comprising a length of detonating cord disposed in a helical or spiral coil as described above, and initiating the detonating cord. Optionally, the detonating cord may comprise low-energy detonating cord and optionally, the receptor charge and the initiator may be substantially free of primary explosive materials.
This invention also relates to an accumulator spindle comprising a spindle body that carries a spiral cut-off barrier, the barrier defining a helical groove on the spindle body; and an anchor aperture. The spindle may also comprise a cleat projection. The helical groove may define a taper angle of from about 2 to 4 degrees. Also optionally, the groove may have two ends and the anchor aperture may be at one end of the groove and the cleat projection may be at the other end of the groove.
Alternatively, the present invention may provide an accumulator spindle comprising a spindle body comprising two spaced-apart parallel plates; an anchor aperture; and a cleat projection.
This invention further pertains to a receptor-initiator assembly comprising a receptor charge comprising a body of explosive material having an initiator well therein; and a helical or planar coil of detonating cord disposed in the initiator well. There may be a receiving portion associated with the body of explosive material, the helical coil may be mounted on a spindle and the spindle may be secured to the receiving portion. For example, the spindle may be secured to the receiving portion by a detent and groove engagement between them. Optionally, the helical coil and the initiator well each define a taper angle of from about 2 to 4 degrees.
In any of the foregoing embodiments, one or both of the initiator and the receptor charge may be substantially free of primary explosive materials.
As used herein, the terms xe2x80x9clargexe2x80x9d and xe2x80x9csmallxe2x80x9d when used to refer to detonating cord, including LEDC, refer to the relative loading of explosive material in the core of the cord, smaller cord having less explosive material per linear unit and, accordingly, a less powerful output than a larger cord.